1. Field of the Invention
The present invention relates to a cathode ray tube having an intermediate electrode between an anode electrode and a focus electrode and is constituted by an electric field expansion type lens as a main electron lens.
2. Description of Prior Art
In recent years, a request to increase of resolution of an electron gun has been more and more intensified. In order to realize the request, there has been proposed a cathode ray tube in which a resistor is built in the tube body, a predetermined voltage is generated by being divided from high voltage and an electric field expansion type lens, an electrostatic quadruple lens or the like is formed without supplying voltage from outside a CRT (Cathode Ray Tube) via a stem pin.
In this case, the build-in voltage dividing resistor may be fixed onto beading glass in which the resistor is formed, for example, on a ceramic substrate and reception and supply of voltage is carried out via a pin installed on the substrate Accordingly, a distance between the resistor and a neck inner wall is proximate only at a location where the resistor is fixed compared with other space.
In the meantime, there has been provided a technology in which a metal line is welded to the outer periphery of a focus electrode of an electron gun, the metal line is heated, a portion thereof is evaporated and a metal film is formed on the neck inner wall, by which the potential of the neck inner wall in the tube is stabilized and intra tube discharge is restrained.
However, in the case of an electron gun having a voltage dividing resistor for high voltage, in view of the structure, a metal line is proximate to a neck inner wall by the amount of the thickness of the voltage dividing resistor and an interval therebetween is reduced. An example thereof is shown by FIG. 6. FIG. 6 is an outline sectional view of a neck portion at the surrounding of a 5-1-th electrode (focus electrode) 5 where metal lines 16 are welded to surround a voltage dividing resistor 12 and beading glass 11. As is apparent from FIG. 6, in the case of the electron gun having the voltage dividing resistor 12, in view of the structure, the electron gun is nonsymmetrical. When the metal line is intended to heat from outside of the neck by high frequency or the like with a purpose of forming a metal thin film on the neck inner wall in order to stabilize the potential at the inner wall of the neck or restrain discharge, the metal line on the side of the substrate is excessively proximate to the inner wall of the neck compared with the opposed side and in the worst case, the metal line is brought into contact with the neck inner wall by which neck crack may be caused and therefore, the heating condition needs to be moderate.
Then, it becomes difficult to form a uniform vapor-deposited film on the inner wall of the tube body by the upper and lower two metal lines and excellent withstand voltage characteristic is difficult to maintain.
In addition thereto, the outer diameter of the electron gun is restricted from concern of approach of the metal lines to the inner wall of the neck which makes difficult large aperture formation of the lens of the electron gun.
Further, according to fabrication steps of a CRT, a knocking step is introduced after finishing a step of exhausting the CRT as a countermeasure for preventing discharge in the CRT. In this step, a voltage substantially twice as much as high voltage applied to CRT in actual operation, is previously applied to an electron gun of CRT before integration by so that which forced discharge is caused among electrodes, among leads and among the electrodes and the neck inner wall in the electron gun. In this way, burrs of metal parts or dirt at inside of CRT or the like is removed such that no discharge is caused under high pressure in the actual operation.
However, in a nocking step of CRT provided with an electron gun having a voltage dividing resistor as disclosed in the specification of Japanese Patent Application No. 9-16767, when high voltage for nocking is applied, since an electrode applied with the high voltage communicates with an intermediate electrode or a focus electrode opposed thereto by the voltage dividing resistor although the voltage dividing resistor is a high resistor, current may be conducted simply through the voltage dividing resistor. Thus only creepage discharge is caused between the electrode applied with high voltage and a holder pin for supplying the divided voltage on the substrate having the resistor and therefore, discharge among desired electrodes is difficult to cause. Then, not only forced discharge is not realized among electrodes and among electrodes and dirt present at inside of the neck, which is the original purpose, but the voltage dividing resistor per se may be destructed by concentrating discharge to the holder pin. Further, depending on cases, the divided voltage applied to a predetermined electrode may be changed by the destruction of the voltage dividing resistor, which may give rise to, for example, deterioration in focus characteristics or the like.